Echo cancellation in long distance telephonic communications is well known in the art. The need for such echo cancellation arises from impedance mismatches associated with wireline telephone subscribers and an economic decision by telephone carriers to use two-wire connections between wireline subscribers and the central telephone offices.
Two-wire connections require mixing of a duplex telephone signal (transmit and receive) for exchange between the central telephone office and the wireline subscriber. The mixing of transmit and receive signals results in a portion of a received signal being re-transmitted as an outgoing signal from a receiving subscriber to a transmitting subscriber. While the re-transmitted signal may be perceived as a "hollow" sound to local communicators, the retransmitted signal may represent a distracting echo in long distance communications.
The delay experienced by a subscriber between a transmission and an echo may be a determining factor in the acceptability and usability of the communication channel. Short delays experienced between local communicators (on the order of 1-20 milliseconds) typically do not represent an impediment to the efficient exchange of spoken words. Longer delays, (on the order of 250-500 milliseconds), however, may result in syllables and even entire words being repeated as an echo and may render the communication channel unusable.
The advent of digital mobile communications systems has exacerbated the problem of time delays, and hence, the need for echo cancellation. Vocoder delays, convolutional coding algorithms, etc. typically introduce round trip signal delays in mobile communication circuits on the order of 200 milliseconds.
The solution to the echo problem has been to provide computer based echo cancellers. Echo cancellers are typically based on adaptive finite impulse filter (AFIR) theory. A comprehensive discussion of AFIR theory is provided in Adaptive Filter Theory, 2nd ed., by Simon Haykin, Prentice Hall, 1991. AFIRs provide for echo cancellation by generating a mathematical model of the echo characteristics of a communication system as a step in canceling the echo.
AFIRs, however, suffered a number of disadvantages including poor filter convergence time and filter instability. The invention of the afore-mentioned U.S. Pat. No. 5,295,136 solved these problems by providing an improved method for converging an adaptive filter echo canceller. The method disclosed provided for identifying the location of a primary echo in the echo filter vector, dividing the vector into primary and secondary vectors and increasing the adaptation rate relative to filter locations proximate to the primary echo. The primary echo has been determined to contain substantially all of the echo energy, and increasing the adaptation rate proximate to the primary echo provides fast filter convergence without instability.
Double-talk correction in echo cancellers, as presently known, is a process designed to inhibit update of the adaptive filter coefficients in an echo canceller, such as disclosed in U.S. Pat. No. 5,295,136, when the "near-end" speaker is talking. If adaptation of the filter vector coefficients is not inhibited upon detection of near-end speech, the filter vector will diverge leading to poor communication quality. Double-talk detectors designed to inhibit adaptation of the echo canceller filter during periods of near-end speech are traditionally based on comparing an estimated power of the received echo signal to some fixed threshold of the maximum estimated transmit power. This can be represented as: EQU r.sub.s (0)&gt;d.sub.th .times.r.sub.x (0).sub.max,
where r.sub.s (0) is the estimated power of the echo signal, r.sub.x (0).sub.max is the maximum estimated transmit power and d.sub.th is a double-talk threshold constant. Once double-talk is detected, adaptation of the echo canceller is inhibited for a "hangover" period. A disadvantage of a fixed threshold double-talk detector is that the threshold must satisfy the worst case echo return loss (ERL), which is usually about 6 dB. However, typical ERL can be much higher and has readily been observed at 22 dB or more. Therefore, significant filter divergence can occur during double-talk, prior to detection, particularly when the near-end speech is at relatively low energy levels compared to that of the far-end.
Testing reveals that enough echo canceller filter divergence occurs prior to the detection of double-talk to cause significant distortion of the echo canceller output. Because of this filter divergence prior to double-talk detection and inhibited filter adaptation during double-talk, the echo canceller output can remain significantly distorted for at least the hangover period and potentially longer as the filter must readapt once double-talk is no longer detected and the hangover period has expired. Therefore, there is a need for a method and apparatus for quickly detecting double-talk so as to limit filter divergence and for correcting the echo canceller filter to account for the presence of double-talk and to limit effects of echo canceller filter divergence occurring prior to double-talk detection.